Magnetic field sensors, in particular optical fiber magnetometers

Abstract

In a fiber optic interferometric magnetic sensor or magnetic gradient detector a magnetic bias field is applied to a magnetically sensitized portion of an optical fiber by means which are driven by optical power whereby a completely passive sensor head arrangement may be achieved. In a magnetic gradient detector optical power from a high power laser (34) may be supplied via an optical fiber (35) to illuminate a solar cell 27 to whose electrical output terminals solenoids (25,26) are connected. A portion of a respective optical fiber (21, 22) which is magnetically sensitized, by for example bonding to a strip of magnetostrictive material (23, 24), is disposed inside each solenoid (25, 26).

We claim: 
     
       1. A magnetic field sensor including a signal mode optical fiber, a portion of which is magnetically sensitised by being associated with magnetostrictive material, and means for generating a bias magnetic field at said portion, which means is optically powered. 
     
     
       2. A magnetic field sensor as claimed in claim 1, wherein said bias magnetic field generating means includes a solar cell, a respective optical fiber for supplying optical power from a high power laser source to illuminate the solar cell and a solenoid in which the magnetically sensitised optical fiber portion is located, the solenoid being connected across the electrical output terminals of the solar cell. 
     
     
       3. A magnetic field sensor as claimed in claim 2, wherein said bias magnetic field generating means serves to provide an a.c. bias magnetic field whose bias frequency is transmitted as the repetition rate of optical pulses transmitted down the high power laser optical fiber. 
     
     
       4. A magnetic field sensor as claimed in claim 1, wherein said bias magnetic field generating means includes a resonant silicon transducer comprising a thin silicon vibratable element to which is attached a permanent magnet, a magnetic circuit in which are disposed the transducer and the magnetiacally sensitised fiber portion, a high power laser source, and a respective optical fiber for supplying optical power from the high power laser source to the transducer, the optical power from the high power lasser source serving to thermally excite the vibratable element and cause it to vibrate and oscillate the permanent magnet, thus providing an a.c. magnetic field which is routed to the magnetically sensitised fiber portion via the magnetic circuit. 
     
     
       5. An optical fiber interfrometric magnetometer comprising a magnetic field sensor including a single mode optical fibre a portion of which is magnetically sensitised by being associated with magnetostrictive material, and means for generating a bias magnetic field at said portion, which means is optically powered. 
     
     
       6. An optical fiber interferometric magnetometer as claimed in claim 5 and including means for passively demodulating the interferometer. 
     
     
       7. An optical fiber interferometric magnetic gradient detector comprising a magnetic field sensor including a single mode optical fibre, a portion of which is magnetically sensitised by being associated with magetostrictive material, and means for generating a bias magnetic field at said portion, which means is optically powered. 
     
     
       8. An optical fiber interferometric magnetic gradient detector as claimed in claim 7 and including means for passively demodulating the interferometer.